Electric resistor heaters and their production



May 15, 1956 F, w, GLASER 2,745,929

ELECTRIC RESISTOR HEATERS AND THEIR PRODUCTION Filed Oct. 1, 1953 IN V EN TOR. 54% 60942 2 ga mz United States Patent ELECTRIC RESISTOR HEATERS AND THEIR PRODUCTION Frank W. Glaser, New York, N. Y., assignor to American Electro Metal Corporation, Yonkers, N. Y., a corporation of Maryland Application October 1, 1953, Serial No. 383,656 6 Claims. (Cl. 20164) This application is a continuation-in-part of my application Serial No. 359,344, filed June 3, 1953.

This invention relates to electric resistor heater bodies, and more particularly to such bodies which utilize molybdenum disilicide as an essential ingredient and to the production of such heater bodies.

It has long been known that molybdenum disilicide MoSiz has unusually desirable properties which would make it an ideal material for electric resistor heater bodies.

However, in the past, electrical heater rod bodies for electric furnaces and the like have, as a rule, been made out of cemented silicon carbide, such as heater being known commercially as Globar bodies. The known silicon carbide heater bodies have a maximum operating temperature of about 1400 C., and they have a resistivity of 2000 to 5000 microhm centimeters because with lower resistivity it is difiicult to supply such heater bodies with the much larger currents that would be required to bring them to the desired high operating temperatures.

Because of its relatively low electrical resistivity of about 22 microhm centimeters at room temperatures, no practical way was found in the past for utilizing molybdenum disilicide MoSiz as electrical heater bodies. Cemented bodies of molybdenum disilicide are very brittle and fragile and proposals to increase the resistivity of molybdenum disilicide MOSiz by additions of molybdenum aluminides, and/or high melting oxides, such as zirconium oxide, thorium oxide, which become conducting at high temperatures, and/or other high melting oxides, such as aluminum oxide, berrylium oxide, silicon oxide likewise failed because these additions result in a further increase in the objectionable brittleness of molybdenum disilicide MoSiz.

One phase of the present invention is the discovery that desirable thin, bendable heater bodies which have a high degree of ductility and lack the objectionable brittleness, may be obtained by subjecting a continuous thin body of ductile molybdenum, such as a thin molybdenum wire, ribbon or tubing to a siliconizing treatment whereby a substantially thick exterior layer of the molybdenum of said body is converted into molybdenum disilicide without substantially impairing the ductility of the interior molybdenum core of said body.

The foregoing and other objects of the invention will be best understood from the following description of exemplifications thereof, reference being made to the accompanying drawings, wherein Fig. 1 is an elevational view illustrating the procedure for producing a heater body exemplifying the invention; and

Fig. 2 is a cross-sectional view of a heater wire exemplifying the invention.

Bodies of pure molybdenum disilicide MoSiz which melts at about 1880 C. can be readily molded from MoSiz powders by powder metallurgy and ceramic techniques. Because of their high stability and resistance to corrosion in oxidizing atmospheres at 1700 to 1800 C., many proposals have been made in the past for using cemented molybdenum disilicide MoSis material in electric resistance heaters for operating at heating temperatures of about 1700 C. However, because molybdenum disilicide MoSiz has a relatively low electrical resistivity of only 22 microhm centimeters at room temperature, heater bodies of this material would have to be made with a very thin cross-section, such as thin tubing or ribbon, which made their use impractical.

In addition, such cemented MoSiz material is very brittle and accordingly, thin heater bodies of silicon carbide are also impractical because of their excessive critical brittleness.

One phase of the invention is the discovery that thin practical heater bodies having an exterior of molybdenum disilicide and which may be bent and lack the objectionable brittleness may be obtained by subjecting a thin structure or formation of ductile molybdenum such as thin, flexible wire, ribbon or tubing of molybdenum, to a siliconizing treatment wherein a relatively thick stratum of the molybdenum structure is converted into molybdenum disilicide MOSiz thereby providing a thin heater body of high flexibility and having a relatively thick exterior integral layer of molybdenum disilicide MoSiz which makes it possible to operate such heater body in oxidizing atmospheres at temperatures above 1400" C. for prolonged periods of time without deterioration.

Contrary to expectations, it has been found that when a thin flexible ductile wire, ribbon, sheet or like formation of molybdenum is subjected to siliconizing in a nonoxidizing atmosphere in such manner as to cause all the molybdenum of a substantial depth of the exterior of the thin formation to be converted into molybdenum disilicide, there is obtained a core of molybdenum having a very dense thick exterior body layer of molybdenum disilicide M0Si2, without materially impairing the ductility of the molybdenum core, and that such formation may be used in thin form as an electrical heater body, for instance, as a ribbon or wire which may be bent and twisted without breaking. Such ribbon or wire having an exterior body layer of molybdenum disilicide MoSiz may be wound as a spiral heater body around a refractory support such as a refractory rod of ceramic or like material.

Any of the known methods for depositing silicon on the surface of a metal body heated to below its melting temperature may be used for forming on the surface of thin molybdenum formations a layer of MoSiz.

A suitable process is one wherein the molybdenum formations are subjected to a gaseous mixture of SiCl and H2 at elevated temperature in the range from about 900 to 1500 C. wherein the silicon of the gaseous mixture is deposited on the surface of the molybdenum formations and combined with the surrace layer thereof into a surface layer of MoSiz of the desired substantial thickness.

Another suitable process is as follows: The molybdenum formations are packed in a pack mass which is effective at elevated temperature in depositing silicon on the molybdenum. The pack may consist, as an example, of 50% by volume, of ceramic lumps, balance silicon metal. Heat-resistant treatment baskets which are so packedare placed in a retort and heated therein to temperatures from 900 to 1100 C. while passing therethrough an atmosphere of pure dry hydrogen or dry hydrogen chloride gas for producing reactions at which silicon of the pack is deposited on the molybdenum and combined therewith into a surface layer of MoSiz.

By way of example, the thin molybdenum formations such as wire or ribbon about .040 to /1 inch thick are placed in a retort and a stream of pure hydrogen and gaseous silicon tetrachloride is passed through the retort while heating the contents to an elevated temperature of 900 to 1160 C. Satisfactory results are obtained with gas mixtures containing from 4 to 30% (by volume) of SiCl4 at rates of 3060 cc. per second for a period from to 30 hours and longer. By way of another example, the thin molybdenum formations are packed in treatment baskets within packs consisting of 50% by volume ceramic lumps, such as porcelain pieces, the balance silicon metal. The treatment baskets are then placed in a retort heated to about 1300 C. While passing therethrough an atmosphere containing pure dry hydrogen and gaseous silicon tetrachloride for producing reactions causing silicon atoms to be deposited on the molybdenum formations and to difiuse into the molybdenum and to combine therewith into molybdenum disilicide MOSiz until all molybdenum of the surface layer of desired or of entire thickness is converted into molybdenum disilicide MoSiz. Good results are obtained with thin molybdenum formations, up to about inch thick, and even up to inch thick, which have been siliconized to a depth of 0.008 to 0.010 inch although siliconizing to a depth of 0.003 to 0.004 gives likewise satisfactory results.

Although the resistivity of the resulting exterior layer of MoSiz of such formation is of the same order as that of cemented molybdenum disilicide MoSiz, the thin molybdenum formation of molybdenum disilicide MoSiz so obtained have great density, and such thin ribbon or Wire having a core of Mo and an exterior body layer of molybdenum disilicide MOSiz may be bent without breaking. Such thin ribbon or wire formations consisting of a core of Mo with an exterior body layer of molybdenum disilicide MoSiz may be Wound in spiral form in the same way as Nichrome wire into spiral heater bodies, and around supports of refractory material, such as ceramic rods, to provide desired resistance heater bodies which will operate for a long period at temperatures of about 1700 C.

As explained above in carrying on the siliconizing process, the thin molybdenum formations such as ribbon or wire are packed Within a silicon pack and while held in the silicon pack is subjected to the siliconizing action which converts all molybdenum into molybdenum disilicide MoSiz. It is good practice to cover the upper side of the pack with a refractory cover or a material similar to that used for the refractory boat of the pack. If tubing of molybdenum is to be siliconized in molybdenum disilicide MoSiz, the silicon metal powder of the pack is placed into the interior of the tubing as well as around the exterior thereof.

A wire or ribbon of molybdenum may be siliconized into molybdenum disilicide MoSiz on a continuous basis by feeding the Wire to a siliconizing equipment wherein it is subjected to a siliconizing action in the manner described above and from which it is withdrawn after performing thereon the siliconizing action.

' Such siliconizing process of the invention is indicated diagrammatically in Fig. 1. A Wire 14 of molybdenum is led into siliconizing equipment 15 wherein it is subjected to siliconizing action in the manner described above and wherein all of the molybdenum on the exterior of the wire up to a depth of at least about 0.003 inch is converted into molybdenum disilicide MoSiz.

, the type described above.

The siliconized wire formation so obtained is indicated at 16 after it is withdrawn from the siliconizing equipment 15. Fig. 2 shows the cross-section of such wire, having a core 31 of molybdenum with an exterior layer body 32 of MoSiz integral therewith and having a thickness of about .008 inch.

Wires of molybdenum having a thickness of about 0.040 to 0.080 inch which have been provided in accordance with the invention with a dense exterior body layer of MoSiz having a thickness of about 0.008 to 0.010 inch may be bent into a circular formation with an inner radius as small as about inch and greater without impairing the protective character of the dense exterior formation of MoSiz for the molybdenum core when such formation is heated to a temperature between 900 and 1500 C. within an oxidizing atmosphere in which an exposed molybdenum Wire or body would otherwise be quickly burned away.

According to another phase of the invention desirable electric heaters are made as follows: A Wire or ribbon formation of molybdenum is wound around a relatively rigid supporting rod or structure of ceramic material, which ceramic structure retains its strength at the elevated temperatures of the siliconizing treatments of Thereafter, the supporting structure with the molybdenum formation mounted on the exterior of the rod are packed into a siliconizing pack and subjected therein to a siliconizing treatment of the type described above until all of the molybdenum formation which is supported on the ceramic body is siliconized into MoSiz having the small cross-section required to give it the desired resistance for limiting the current required for heating it to the desired high temperature.

Heaters of the invention formed in the manner described above are superior to conventional silicon carbide heaters because they may be operated for prolonged periods of time without corrosion at temperatures much higher than the maximum temperature of 1400" at which such silicon carbide heaters have to be used.

The features and principles underlying the invention described above in connection with specific exemplifi cations, will suggest to those skilled in the art many other modifications thereof. It is accordingly desired that the appended claims be construed broadly and that they shall not be limited to the specific details shown and described in connection with exemplifications thereof.

I claim:

1. In an electric heater device, an elongated self-supporting mounting member of electrically insulating hard refractory material, a helically coiled electrically conductive heating structure surrounding with its coil turns said mounting member and supported thereby, said coiled heating structure having a length that is much greater than its maximum cross-sectional dimension, the entire exposed exterior of said structure consisting essentially of a body layer of molybdenum disilicide MoSiz having a thickness of at least 0.003 inch, and the interior of said structure consisting essentially of molybdenum.

2. In an electric heater device as claimed in claim 1, said structure having the shape of a ribbon.

3. In an electric heater device as claimed in claim 1, said structure having the shape of a wire.

4. In an electric heater device as claimed in claim 1, said body layer of MoSiz having a thickness of at least 0.008 inch.

5. In an electric heater device, an elongated self-supporting mounting member of electrically insulating hard refractory material, a helically coiled electrically conductiveheating structure surrounding with its coil turns said supporting structure and said mounting member and sup ported thereby, said heating member having a cross-sectional area corresponding to a circle with a diameter from about 0.040 to 0.250 inch, the entire exterior body layer of said elongated heating member having a thickness of at least 0.003 inch and consisting essentially of molybdenum disilicide MoSiz, the interior core of said heating structure consisting essentially of molybdenum.

6. In an electric heater device as claimed in claim 5, said body layer of MoSiz having a thickness of at least 0.008 inch, said interior having an area corresponding to a circle with a diameter of at least 0.030 inch.

References Cited in the file of this patent UNITED STATES PATENTS Marshall Apr. 12, 1932 Becker Apr. 6, 1948 Campbell et a1. Jan. 12, 1954 Campbell Jan. 12, 1954 FOREIGN PATENTS Great Britain Dec. 19, 1949 

1. IN AN ELECTRIC HEATER DEVICE, AN ELONGATED SELF-SUPPORTING MOUNTING MEMBER OF ELECTRICALLY INSULATING HARD REFACTORY MATERIAL, A HELICALLY COILED ELECTRICALLY CONDUCTIVE HEATING STRUCTURE SURROUNDING WITH ITS COIL TURNS SAID MOUNTING MEMBER AND SUPPORTED THEREBY, SAID COILED HEATING STRUCTURE HAVING A LENGTH THAT IS MUCH GREATER THAN ITS MAXIMUM CROSS-SECTIONAL DIMENSION, THE ENTIRE EXPOSED EXTERIOR OF SAID STRUCTURE CONSISTING ESSENTIALLY OF A BODY LAYER OF MOLYBDENUM DISILICIDE MOSI2 HAVING A THICKNESS OF AT LEAST 0.003 INCH, AND THE INTERIOR OF SAID STRUCTURE CONSISTING ESSENTIALLY OF MOLYBDENUM. 